Amplifier of electrical currents



Sept. 4, 1928; 1,683,083

E. H. LOFTlN AMPLIFIER OF ELECTRICAL CURRENTS Filed June 22, 1923 Patented Sept, 4, 1928. i v

UNITED STATES".

i EDWARD H. LOFTIN, OF WASHINGTON,

-DISTRIGT or COLUMBIA, 'assrenoa r EDWARD n. 1.0mm, TRUSTEE.

AMPLIFIER OF ELECTRICAL GURRENTS.

Application filed June 22,

While my invention relates generally to amplifiers of electrical currents, aparticular object is the simultaneous amplification of high frequency and low frequency electrical currents using the same amplifying devices and circuits in whole or in part for the double function.

Another object of my invention is to .obtain a high order of amplification using a 0 minimum of apparatus.

' A furtherobject is to obtain stable operation of multiple stage amplifiers, particularly in those types in which the same amplifying devices and circuits are used for radio and 5 audio frequency amplification, through avoiding production of oscillations within the circuits or regenerative efiects to the point of distortion, and to do so without the use of. screened compartments for the several stages 3 of amplification, and Without the use of potentiometer stabilizers, neutralizers and like complications. 4

A further object is avoiding the use of radio frequency by-passes around audio frequency- 5 devices (such as transformers) hitherto considered essential, and taking advantage of a beneficial effect on amplifier circuitsthereby secured.

Another object is to provide for the use of loud speaking devices as translating elements without additional amplifying devices for audio frequency. amplification.

That part of my system which has to do with using the same amplifying devices and i circuits for the double function of amplify ing radio frequency and audio frequency currents is in general accomplished in a manner illustrated in U. S. Patent 1,087,892 of February 17, 1914, to Schloemilch, et al, Figure 4, but I do not employ by-pass condensers such as that illustratedat S in shunt with transformer winding 0 in the input circuit. I also refer to British Patent 131,092, of August 21, 1919, and point out that I do i not use condensers such as 12 and 13 in shunt with transformer winding 9. Also to British Patent 132,668 of September 25, 1919, further pointing out that I do not use condensers such as 17 and 18 in shunt with transformer 13 and 20 and 21 in shunt with transformer 14.

My invention will be best understood by reference to the'figure in the accompanying drawing in which I illustrate 3 three-electrode thermionic vacuum tubes having cir- 3 cuit connections for amplifying, the three 1923. Serial No. 647,027.

tubes being employed in tandem as radio frequency amplifiers, indicated by reference des gnations VT VT and VT,, and two of the tubes namely VT, and VT being further employed as amplifiers of audio frequency currents. The system is illustrated as em ployed to, amplify radio signals collected on the antenna A having ground connection G, and tuning elements inductance L and variable capacity 0,. The collected signals are transferred to the amplifier throu h the secondary circuit having inductance 2 coupled to inductance L and variable capacity (3 A source of energy for the plate circuits of x the vacuum tubes is impressed across the ,lin'es 1 and 2 so that line 1 is the positive side of the source. A source of energy for heating the filaments for the vacuum tubes is impressed across the lines 2 and 3, and by making the line 3 thenegative side of this source a negative potential or bias is obtained on the grids of the several vacuum tubes, thereby causing them to function as amplifiers. A rheostat to control the amount of heating of the filaments of the vacuum tubes. RF RF, and RF are radiofrequency transformers linking the several stages of the system together totransfer radio frequency currentsfrom one to the other. AF and AF areaudio frequency transformers to impress audio frequency currents on the system and pass them from one stage to another. D is any suitable form of rectifying device to aid in rev ducing modulated radio frequency currents to low frequency currents characteristic of the modulations, and may be a crystal re'cti-' 'may be inserted in either of the lines 2 or 3 fier, a vacuum tube rectifier, an electrol tic of audio frequency transformer AF and the primary and secondary windings of the audio frequency transformer AF, are not shunted with condensers to provide low impedance paths for radio frequency currents around these devices, as is done in the art heretofore referred to; that is, these devices offer their natural impedances to the currents.

While I have illustrated the radio frequency and audio frequency transfer devices as rtransformers, it is not essential to the invention that they be of this form, as auto transformers or other suitable forms may be used; however, I prefer the transformer type as itsimplifies the method of connection to prevent impressing the high potential of the plate circuits on the grids of succeeding tubes. The transformers show are of the band type; that is, they are designed to amplify over a band of frequencies rather than being sharp ly resonant to a particular frequency, though m a special design or by adjustment where extreme selectivity is esired transfer elements sharply resonant to one frequency could be employed, as indicated by the vari-- able condensers shown in dotted lines connected in a well knownmanner across the secondary winding of transformers RF, and RF,. The operation of the system is as follows:

Radio frequency currents modulated at audio frequencies are collected on the antenna A and transferred to the amplifying system through the secondary circuit containinginductance L and capacity C -The radio frequency currents are then amplified ste by step by the vacuum tubes 'VT VT and V the currents being transferred from stage to stage by'the radio frequency transformers RF and RF,. The currents having undergone three stages of amplification are passed into detector circuit containing the detector D through radio frequency transformer RF In the audio frequency transformer AF 'in the detector circuit the restified currents are converted into audio frequency currents characteristic of the modulations of the radio frequency current. These low frequency currents are then impressed upon vacuum tube VT through audio frequency transformer AF,, and thence to vac uum tube VT through audio frequency transformer AF,, and having undergone two stages of audio frequency amplification. are translated in the loud speaking devices LS.

The radio frequency transformers do not offer any appreciable impedance to the flow of audio frequency electrical currents as the windings have few turns of wire, or wire of little length, compared to the windings of' the audio frequency transformers, and in many cases do not have iron cores. In those radio frequency transformers having iron cores the iron employed has such magnetic characteristics andphysical dimensions as not to appreciably influence the impedance to audio frequency currents. On the other hand the impedance of the audio frequency i,ess,oes

which tendency comesabout through reactive effects of the various circuits upon each other through the coupling between plate and grid circuits due to internal capacities of the vacuum tubes, and is also particularly strong in an arrangement of this kind where a feedback is introduced through the return connection for audio frequency currents. It has heretofore been considered necessary to provide by-pass condensers around the windings of the audio frequency transformers for the passage of radio frequency curents, thus rendering the effective capacitive impedance of these devices to radio frequency currents non-- appreciable. But such provision destroys the utility of these windings as choking devices for oscillations and regeneration to -a point of distortion. v

. While, by leaving out radio frequency bypasses, I render the impedance of the audio frequency devices to radio frequency cur-' rents large, yet I do not materially decrease the amplifying ability of the system. In the first place, while the radio frequency impedance is made relatively large as compared to a by-pass device, it still remains considerably less than the impedance of the radio frequency transformer to radio frequency currents, as the radio frequency transformer may be constructed or adjusted to be exactly in resonance with these currents, or nearly so in the band type of transformer, and a resonant or near resonant circuit offers an extremely high impedance to resonant currents, probably of the order of a hundred thousand ohms, while the. audio frequencytions andreactions that inevitably obtain in thesystem disclosed and taking into consid eration well-known effects in such systems.

Considering tube VT,,, for instance, and its input and output-circuits, if the transformers RF and RF are of the band type they do not have sharply defined resonance peaks, but nevertheless exhibit resonant characteristics, and therefore react inductively for current frequencies on one side of resonance and capacitively for current frequencies on the other side of resonance. 7 Since an inductivelyreacting output circuit produces a feed back through the plate-torgrid capac-' ity of a three electrode vacuum tube-that is favorable to regeneration or oscillation production',it isobvious that when the system is adjusted or used at frequencies on the inductive'ly reacting side of the natural resonant frequency of transformers RF, and RF,, as

by adjusting tuningcondenser O there is a tendency for tube VT to cooperate with its input and output circuits to produce oscillais always at hand a substantial capacitive reaction from the audio transformer to modify or neutralize the inductive reaction, 'and'this reaction is therefore not effective to produceoscillations. If thelcapacitive reaction of the audio transformer is substantially reduced,

"as by shunting with a condenser, as has been the practice, the troublesome inductive reaction of the radio frequency transformer is not sufliciently overcome to prevent oscillation if the tubes are sufficiently energ zed tobe use- By not shunting the secondary winding of audio frequency transformer AF with a, condenser its capacitive reaction to rad o frequency currents is maintained large also, and.

this capacitive reaction brings about a substantial change of phase of the in t circuit currents, with the resultthat if all of the in:

ductive reaction of the output .circuit'is not neutralized, permitting a residual reaction acting through the grid-to-platecapacity favorable to regeneration, the-dephased input tivness of this feed back. 4 I

The effect of substituting tunable circuits for the band transformers is to make the rescircuit currents assist in reducing the effeconant peaks movable throughout the wholerange of tuning, so that the. troublesome output circuit inductive reactions can be produced anywhere withinthe range-of tuning provided, but'since the reactions of the audio transformers are capacitive throughout any usual range of radio frequency operation, the

capacitive reactions are always present to modify the efiects of the inductive reactions, and thus prevent-oscillations. throughout a wide range in a variable tunable system.

I'have constructed the identical com-binationjillustrated in the figure without any precautions whatsoever as to shielding or screenmg one stage of amplification from another,

without any precautions as to spacing of the transformer elements, wiring and other parts,

and without any shield forthe loud speaker leads, andfhave found the arrangement extremely efficient and effective and absolutely stable and u'ninfluenced by movement of hands about the various parts, or movement of the loud speaker and its leads in the immediate vicinity of 'amplification'circuits. I

have also found the reproduction of voice and music to be totally without distortion.

Having described my invention 1 claim 1. A stage of amplification inan amplifier system including a three-electrode vacuum tube, an input circuit having a portion tuned to currents above audibility, means for impressing currents above audibility on said tuned portion, means'associated with said input circuit for simultaneously impressing substantially lower frequency currents on said input circuit, an output circuit, means for transferring said currents above audibility from said output circuit, and .means for simultaneouslytransferring said lower frequencycurrents from said output circuit including a coil in said output circuit so connected as to unmodifiedly offer its natural impedance tothe flow of said currents above audibility insaid output circuit. I

2. A stage of amplification in an amplifier system includinga three-electrode vacuum tube, an input circuitj having a portion tunable to high frequencyelectrical currents,

an output circuit" having an associated reactance system adapted to be tuned in consonance with. the tunable portion in. said input circuit, thereby producing high frequency reactions through the internal capacity 'of said tube in phase with currents in saidinput circuit, acoil in said platecircuit adapted through having high inductance and lowv distributed capacity to substantially alter the high frequency reaction o-fsaid outsaid input circuit in series-with said tunable portion.

5 3. A stage of amplification in an amplifier system including a three-electrode vacuum tube, an;input circuit including a circuit tunable to high frequency currents, an output circuit having an associated circuit tunable in consonance with said tunable circuit in said input circuit,'and means in both saidput circuit, and a conductive impedance in input and output circuits having suflicient capacitative reactancjes to effectively alter the reactions-in said circuits of said tunable cir- 'cuits to said high frequency currents,-whereby oscillation by reason of reaction through vented. a

I 4; A stage of amplification in an amplifier system including a three-electrode vacuum the internal capacity of said tube is pretunable to high frequency currents, an output circuit having an associated clrcuit tunable in consonance with said tunable circuit in said input circuit, and audio frequency transformers associated with said inputand output circuits so connected therein that their capacitative reactances to high frequency currents alter the reactions of said tunable circuits to sufficient degree to prevent oscillation by reason of reaction through the internal capacity of said tube.

5. A stage of amplification in an amplifier system including a three-electrode vacuum tube, an input circuit including a circuit tunable to high frequency CllIIBIltS, means for impressing'high frequency currents on said tunable circuit, means for simultaneously impressing audio frequency currents on said input circuit, an output circuit having an associated circuit tunable to hi h frequency currents in consonance with the tunable circuit in said input circuit, and an audio frequency transformer connected in said output circuit to transfer amplified audio frequency currents therefrom, and so connected as to unmodifiedly offer the whole of its inherent capacitative impedance to amplified high frequency currents in said output V circuit, whereby regenerative reaction through the internal capacity of said tube is limited to suflicient degree to prevent oscillation. c

EDWARD H. LOFTIN. 

